FIG. 12 is a diagram illustrating an example of a light-emitting device. The light-emitting device illustrated in FIG. 12 includes at least one light-emitting element (light emitting diodes in FIG. 12) Z1, and a light-emitting element driving device that drives the light-emitting element Z1.
In the light-emitting element driving device included in the light-emitting device illustrated in FIG. 12, a control unit 3 controls an upper-side driver 2H and a lower-side driver 2L to turn on and off N-channel MOS field-effect transistors 1H and 1L (hereinafter referred to as an upper-side transistor 1H and a lower-side transistor 1L), in accordance with a reset signal RESET output from a comparator COM1 and a set signal SET output from a comparator COM2. In this way, a voltage (switch voltage Vsw) at a connection node between the upper-side transistor 1H and the lower-side transistor 1L is generated.
When output current Io that flows in the light-emitting element Z1 exceeds a threshold value, the reset signal RESET output from the comparator COM1 rises, and just after that the output current Io decreases so that the reset signal RESET falls.
An anode voltage Vom of the light-emitting element Z1 is integrated by an RC integral circuit constituted of a resistor ROFF and a capacitor COFF. When the integrated value exceeds a reference voltage VREF, the set signal SET output from the comparator COM2 rises, and just after that the capacitor COFF is discharged by a discharge circuit (not shown) so that the set signal SET falls. After that, when the reset signal RESET output from the comparator COM1 rises, the discharge of the capacitor COFF by the discharge circuit (not shown) is finished.
When the set signal SET rises, the control unit 3 turns on the upper-side transistor 1H and turns off the lower-side transistor 1L. In addition, when the reset signal RESET rises, the control unit 3 turns off the upper-side transistor 1H and turns on the lower-side transistor 1L.
FIGS. 13A and 13B are time charts for describing an operation example of the light-emitting device illustrated in FIG. 12, in which the switch voltage Vsw, the output current Io, the reset signal RESET, and the set signal SET are shown in order from top to bottom. FIG. 13A is a time chart in the case where the number of elements connected in series in the light-emitting element Z1 is large so that the anode voltage Vom of the light-emitting element Z1 is high. FIG. 13B is a time chart in the case where the number of elements connected in series in the light-emitting element Z1 is small so that the anode voltage Vom of the light-emitting element Z1 is low.
An off-time tOFF that is a low level period of the switch voltage Vsw can be expressed by the following equation (1). Note that, in the following equation (1), ROFF, COFF, and VCOFF respectively represent a resistance of the resistor ROFF, a capacitance of the capacitor COFF, and a voltage across the capacitor COFF (i.e. an integrated value of the anode voltage Vom of the light-emitting element Z1).
                              [                      Mathematical            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                t          OFF                =                              -                          R              OFF                                ×                      C            OFF                    ×                      ln            ⁡                          (                              1                -                                                      V                    REF                                                        V                    COFF                                                              )                                                          (        1        )            